1.Field Of The Invention;
The invention relates to a charge pump for supplying at an output a first current in response to a first control signal and sinking at the output a second current in response to a second control signal.
The invention also relates to a frequency synthesis circuit comprising a charge pump.
The invention further relates to a receiver comprising such a frequency synthesis circuit
2. Description Of The Related Art;
A charge pump according to the preamble is known from the document SP24.5 by Akbar Ali and Joo Leong Tham, entitled "A 900 MHz Frequency synthesizer with integrated LC voltage-controlled oscillator", 1996 IEEE International Solid-State Circuits Conference, p. 390-391. The known charge pump is a building block of a phase-locked loop for sourcing a current in response to an up-signal, and sinking a current in response to a down-signal, the current being supplied to a loop filter. In the known charge pump, the up-and down-signals (also called first and second control signals) operate switched current sources. As the switched current sources are placed in cascode, the current sources are customarily implemented using transistors of a different conductivity type. As transistors of different conductivity have different properties, such as a transition frequency f.sub.T, this results in an asymmetric behavior of the charge pump. For example, the first switched current source usually comprises PNP transistors and the second current source usually comprises NPN transistors. In standard IC technology, the PNP transistors are notorious for their relatively poor high frequency performance and, consequently, they cannot be used at the same high frequency as NPN transistors with the same performance. In the known charge pump, this problem is solved by matching the high frequency performance of the NPN current sources with the high frequency performance of the PNP current source. This is done by placing a capacitor parallel to the NPN current source to reduce the high frequency performance of the NPN current source. As a consequence, the maximum frequency of the up- and down-pulses is restricted.